Structural changes in postsynaptic densities may underlie long-term modifications of synaptic activity. The aim of this project is to study the molecular organization of the postsynaptic densities and to explore the potential mechanisms for their modification in response to calcium and other intracellular messengers. Results obtained during the past year using postsynaptic density preparations from cerebral cortex indicate that the calcium calmodulin-dependent protein kinase, which is the most abundant protein in the densities, is a strong candidate for mediating such structural modifications. We show that the densities contain an endogenous phosphatase of type 1 that actively dephosphorylates the calcium calmodulin-dependent protein kinase. When this phosphatase is effectively inhibited, almost all of the postsynaptic density-associated kinase pool becomes phosphorylated. Modification of the postsynaptic density structure in response to elevated levels of calcium may also occur through selective degradation of component proteins by calpain,a calcium-activated protease. Preliminary studies that show different rates of degradation of different component proteins upon treatment of the postsynaptic density preparations with exogenous calpain, support this hypothesis. In the coming year, we shall continue studies on the biochemical and morphological characterization of postsynaptic densities. To study the consequences of the autophosphorylation of the kinase, we shall take advantage of our findings on the use of the phosphatase inhibitor, Microcystin-LR, to obtain a postsynaptic density preparation with a fully phosphorylated pool of calcium calmodulin-dependent protein kinase. The consequences of autophosphorylation and of limited degradation of components by calpain on morphology, protein-protein interactions, and other biochemical properties of the postsynaptic densities will be investigated.